1. Field of the Invention
The present invention relates to a flow meter for measuring flow rate of a gas. More particularly, the present invention relates to an apparatus and method in which the time required to pressurize a chamber with a gas is used to determine the flow rate.
2. State of the Art
A thermal flow sensor is one example of a device used to determine the flow rate of a fluid. The mass flow rate of a fluid is known to be proportional to the amount of heat required to elevate the fluid temperature by a fixed amount as it flows through a laminar flow channel. A heating device and a temperature detector that reacts to the temperature of the heating device are used. The cooling effect on the flow sensor caused by the inflowing fluid is used to determine the flow rate. The greater the velocity of the flow and, consequently, the flow rate of the fluid, the greater the quantity of heat that is taken away from the flow sensor by the fluid per unit of time through heat transfer and convection. Thus, if a constant heating power is supplied to the heating device, the temperature detected by the temperature detector is lower in the case of a high flow rate than in the case of a low flow rate. Accordingly, if the heating power is regulated in such a way that the flow sensor is at a constant temperature, a higher heating power is required in the case of a greater flow rate than in the case of a lesser flow rate.
Calibration is required in order to permit measurement of absolute flow quantities with such a flow sensor. This requires taking into account substantial contributing factors in the determination of heat dissipation capacity, such as effects of the geometry of the flow sensor and the material properties of the fluid. Therefore, accurate use of such a thermal flow sensor requires that the composition of the fluid, the flow rate of which is to be determined, must not vary or must vary only within very narrow limits. Further, the composition of the fluid must be known, and the thermal flow sensor must be calibrated each time the flow rate of a fluid having a different composition is to be determined.
The “rate of pressure rise” or “rate of rise” method may be used to obtain accurate measurements of gas flow rate. Gas flow rate is a measure of the volume of gas that passes a point in an industrial system during a given period of time. Using the ideal gas law, the flow rate may be calculated from the measured values of the rate of change in temperature and pressure of a gas in a chamber of known volume.
This method has been used to test accuracy and calibrate a mass flow controller. A gas flow is conducted through a device under test, such as a mass flow meter, into an evacuated, volume-calibrated chamber for a measured interval of time. The changes in pressure and temperature of the gas in the chamber during the measured interval of time. The gas flow rate can then be calculated from the change in pressure over time and the change in temperature over time in the known volume. The “rate of rise” method requires multiple measurement devices in the gas flow line, and additional hardware, such as cables, connectors and analog/digital converters, increasing the complexity, size, and expense of a gas flow measurement system. Additionally, the flow of gas through the system is interrupted when the flow is diverted to the volume-calibrated chamber.
In view of the above-enumerated deficiencies in the state of the art with respect to mass flow meters, it would be desirable to develop a gas flow meter that does not require separate calibration for each gas or gas mixture and maintained the flow through the system. A flow meter that gives accurate values for low flow rates of mixed-composition gas, as well as pure gas systems, would also be desirable.